x86: doc: Add note about the debug FSP usage on BayTrail

The debug FSP image is bigger in size than the normal FSP image. This
patch adds a small description on how to use this FSP debug version
by changing CONFIG_FSP_ADDR.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

doc/README.x86

@@ -314,6 +314,10 @@ Offset   Description         Controlling config
 
 Overall ROM image size is controlled by CONFIG_ROM_SIZE.
 
+Note that the debug version of the FSP is bigger in size. If this version
+is used, CONFIG_FSP_ADDR needs to be configured to 0xfffb0000 instead of
+the default value 0xfffc0000.
+
 ---
 
 Intel Galileo instructions for bare mode: